1. Field of the Invention
This invention relates to an LD/PD module for optical communications, in particular, aims at proposing a low-cost, small sized LD/PD module. Sometimes a term xe2x80x9coptical communication devicexe2x80x9d is used for signifying an LD/PD module, an LD module and a PD module collectively in this description. LD/PD modules having both a transmitting (LD) part and a receiving (PD) part are suffering from the problem of the invasion of electric and optical signals of the transmitting portion into the receiving portion. The phenomenon of the flow of electrical/optical signals from the LD to the PD is called xe2x80x9ccrosstalkxe2x80x9d. The signals generated by the LD are noise for the PD portion. The crosstalk should be excluded from the PD portion.
This application claims the priority of Japanese Patent Application No.2000-372295 filed on Dec. 7, 2000 which is incorporated herein by reference.
The crosstalk includes electrical crosstalk and optical crosstalk. Size-reduction of LD/PD modules enhances the crosstalk by bringing the PD closer to the LD. Miniaturization of LD/PD modules requires exclusion of the electrical/optical crosstalk. The purpose of the present invention is to propose low-cost, compact optical communication modules by suppressing the optical/electrical crosstalk.
2. Description of Related Art
FIG. 1 shows one of the most prevalent LD/PD modules. A laser diode (LD) as a light source is stored in a cylindrical, metallic package. A photodiode (PD) is also stored in another cylindrical, metallic package. The LD module 1 and the PD module 2 are connected via optical fibers 3 and 4 with a central station (not shown in the figures). Pins 9 and 9 fix the LD module 1 and the PD module 2 to a print circuit board 5 and connect the LD 1 and the PD 2 to some of the wiring patterns on the board 5. The print circuit board 5 maintains a transmitting circuit 6 and a receiving circuit 7.
A metallic shield plate 8 stands at a boundary between the transmitting circuit 6 and the receiving circuit 7 for suppressing electric crosstalk from the transmitting circuit 6 to the receiving circuit 7. The metallic shield plate 8 which is grounded (connected to the earth level on the circuit board) absorbs electromagnetic noise. The metallic packages prohibit light of the LD 1 from leaking and forbid the PD 2 from receiving the LD light. The LD/PD device of FIG. 1 succeeds in lowering electrical/optical crosstalk between the LD and the PD.
The LD/PD device having a discrete structure of FIG. 1 has drawbacks of forbidding further size/cost-reduction. The metal-packaged LD module 1 and the metal-packaged PD module 2 are large and expensive. The print circuit board 5 for mounting the transmitting circuit 6 and the receiving circuit 7 is wide. The discrete LD/PD module of FIG. 1 has attained to the limit of reducing the size and alleviating the cost. Still further size/cost reduction of LD/PD modules is indispensable for the prevalence of the optical communications systems.
A promising candidate is a planar type module preparing a silicon bench having V-grooves and an insulating overcoat, producing metallized wiring patterns on the silicon bench, mounting PD/LD chips on the patterns and fitting optical fibers into the V-grooves for facing the front ends to the PD and the LD. The planar type module is proposed by, e.g.,
{circle around (1)} R. Takahashi, K. Murakami, Y Sunaga, T. Tokoro, M. Kobayashi, xe2x80x9cPackaging of optical semiconductor chips for SFF optical transceiverxe2x80x9d, PROCEEDINGS OF THE 1999 ELECTRONICS SOCIETY CONFERENCE OF IEICE, C-3-28, p133 (1999).
FIG. 2 shows the plan view of the module proposed by {circle around (1)}. A SiO2 insulating layer 11 is made upon a rear portion of a flat silicon substrate 10. Two parallel V-grooves 12 and 13 are formed upon a front portion of the silicon substrate. A transmitting fiber 14 and receiving fiber 15 are fitted into the V-grooves 12 and 13 on the substrate. Metallized patterns 16, 18 and 19 are printed upon the insulating layer 11 of the substrate 10. An LD chip 22 is mounted upon the pattern 18. A PD chip 23 is mounted upon the pattern 19. A monitoring PD 70 is fitted upon the pattern 16 behind the LD 22.
The LD makes transmitting light signals S which are in proportion to the driving current. The LD chip emits the light signals from both the front end and the rear end. The forward transmitting light signals S propagate in the transmitting fiber 14 to the central station. The rear light is detected by the PD 70 which always monitors the average power of the LD. The receiving light signals R which have been generated in the central station propagate in the receiving fiber 15 and go into the PD 23. The PD 23 makes photocurrent which is in proportion to the receiving light signals. An upper half above a dotted line 26 is a transmitting portion (B) and a lower half below the dotted line 26 is a receiving portion (C). Both the portions B and C are built upon the common silicon substrate 11 which is far smaller than the print circuit board 5 of FIG. 1.
The planar type module has advantages over the discrete one of FIG. 1. The V-grooves 12 and 13, the PD/LD mounting patterns 18 and 19 and the monitoring PD mounting pattern 16 can be made on the silicon bench 10 at a stroke. The fibers 14 and 15, the LD and the PD can be exactly positioned by the grooves or marks without positive alignment, which is called xe2x80x9cpassive alignmentxe2x80x9d. Unification of the tiny silicon bench allows the module to reduce the size and the cost. The size/cost-reduction will raise an industrial value of the tiny on-silicon planar module.
The planar module mounting electronic devices, optoelectronic devices and fibers in two dimensions on the bench are an excellent and promising technique. The use of the silicon single crystal substrate enables device makers to employ the well-matured silicon photolithography technics. The photolithography can either form the V-grooves with a narrow spacing on the silicon substrate and position PDs and LDs at the determined spots with preciseness.
On the contrary, the PD module and the LD module of FIG. 1 require active alignment of searching the optimum spots of the PD and the LD by supplying current to the LD, guiding the LD light via the fiber to the PD, measuring the power, displacing the LD and maximizing the output power. The active alignment which should determined the optimum spots for the LD with a xc2x11 xcexcm tolerance and for the PD with a xc2x15 xcexcm tolerance requires much time and high cost.
The planar type module shown by FIG. 2 which mounts the fibers and the device chips on the silicon bench dispenses with the time-consuming, costly active alignment. The module determines the positions of the optoelectronic devices (LDs, PDs) by referring to the marks printed on the Si bench (substrate) and the positions of the fibers by the grooves. The mounting mode without the alignment step is called xe2x80x9cpassive alignmentxe2x80x9d. The planar type module has a possibility of alleviating the cost by eliminating the time-consuming alignment.
The passive alignment enables the planar type modules to position optoelectronic devices (LDs, PDs) and electronic devices (AMPs) to the optimum spots with sufficient coupling efficiency by referring to the marks printed on the silicon substrate without lightening the LDs and detecting the light by the PDs. The planar type communications modules are promising technology which will accomplish high precision mounting without active alignment.
Another advantage of the planar type modules is the grooves formed on the silicon substrates for mounting optical fibers. In the case of the LD/PD modules which exchange signals with e.g., the central station via two parallel optical fibers, formation of V-grooves which has the spacing equal to the standardized fiber spacing will enhance the productivity and cut the cost down. Despite the promising prospects, the planar type optical communication modules have not been put into practice yet. They still stay at the experimental steps. Several causes deprive the proposed planar modules of the reality. This invention takes up the crosstalk among the causes.
The discrete type LD/PD module of FIG. 1 electrically shields the receiving portion C with the metallic packaged PD from the transmitting portion B with the metallic packaged LD by the middle metallic shield plate 8. There is a long distance between the LD and the PD. Triplet metallic shields and the long distance perfectly suppress the crosstalk from the transmitting part B to the receiving part C.
The planar unified type LD/PD module of FIG. 2 has neither metallic shield plate on the middle dotted line 26 nor metallic packages storing the LD and the PD. The silicon substrate allots a poor distance to the LD and the PD. The use of double fibers alleviates the optical crosstalk. But the non-metallic shield and the short distance incur large electric crosstalk in the planar type of FIG. 2. Electric crosstalk degrades the performance of the LD/PD module by impeding the PD from receiving correct signals (R) propagating in the fiber 15.
When the optical communications device contains a pair of a receiving part and a transmitting part or a plurality of receiving parts, the crosstalk causes a serious problem on the receiving parts. The crosstalk is an inherent problem, in particular, in LD/PD modules which contain both the transmitting parts and the receiving parts. The transmitting portion generates strong pulse current signals with a high repetition rate of a low impedance for injecting the current into the LD. The receiving portion has a high input impedance for receiving weak signals with high sensitivity. The strong signal currents leak from the transmitting portion to the high impedance-receiving portion as noise, which is the electric crosstalk.
It is believed by the ordinary skilled that the electrical crosstalk should be annihilated by building a metallic plate between the transmitting portion (B) and the receiving portion (C) and by connecting the metallic plate to the ground (earth level) of the wiring pattern. If noise with a high pulse repetition rate makes a direct flight as electromagnetic waves from the transmitting portion (B) to the receiving portion (C), the tall metallic plate would effectively absorb the electromagnetic noise and protect the receiving portion (C) from the noise. Such a contrivance is suggested by,
{circle around (2)} Sonomi Ishii, Takehiko Nomura, Atsushi Izawa, Masayuki Iwase, xe2x80x9cCrosstalk analysis of MT-RJ optical Sub Assemblyxe2x80x9d, PROCEEDINGS OF THE 2000 ELECTRONICS SOCIETY CONFERENCE OF IEICE, SC-3-7, p352 (2000).
The optical sub-assembly proposed by {circle around (2)} contains a silicon bench (substrate), a PD and an LD mounted on the silicon substrate, fibers facing the PD and the LD, and a metallic plate standing between the PD and the LD for shielding the PD from the LD. The assembly lacks an amplifier and a driving IC. The metallic plate is connected to the ground (earth) of a metallized wiring pattern on the silicon bench. The metallic plate absorbs the electromagnetic noise from the transmitting part.
The electrostatic shield is a stereotype solution for protecting a circuit from external electromagnetic noise.
The transmitting portion of the sub-assembly {circle around (2)} contains only the LD but lacks the driving IC which will inject a large current to the LD and will make strong electromagnetic noise. Thus, the noise made by the transmitting portion is weak. The metallic plate standing in the middle of the circuit would be sufficient to annihilate the weak noise.
If the transmitting part also included a driving IC for supplying the injection current to the LD or other strong noise (current or electromagnetic waves) generators, the crosstalk would be more serious and more critical than {circle around (2)}. The brusque metallic plate would be insufficient for excluding the crosstalk.
Print circuit boards (made from e.g., insulating epoxy resin) have been the most popular boards for printing copper wiring patterns and mounting electric devices. The grounded metallic shield has been a common technique for suppressing noise on the epoxy circuit boards, as shown in FIG. 1. The inventors of the present invention were aware of a new problem originated from the displacement of the print circuit board on a silicon substrate. The silicon crystal is an excellent material for the substrate of electric circuits, because photolithography can make microscopic structures on the single crystal silicon substrate. The skilled may deem silicon substrates as an equivalence of the traditional epoxy print circuit boards. But it is not true. Silicon crystal is not an equivalence to the epoxy circuit boards as a substrate.
The SiO2/Si substrate which is employed in the planar type modules of FIG. 2 has an inherent problem. The epoxy print circuit board is an insulator. But silicon is not an insulator but a semiconductor. Then, an insulating layer, for example, silicon dioxide (SiO2) is formed upon the silicon substrate for mounting electric devices and printing wiring patterns. The conductive silicon is coated with the insulating SiO2. The surface of the silicon substrate is insulating. Thus, the skilled deem the SiO2/Si substrate as a superior brother of the plastic (epoxy) circuit boards. The overcoating insulating layer (SiO2, SiN and so on) is very thin. The thin insulator can cut DC current but cannot cut high frequency AC current. The SiO2/Si substrate conveys AC current by acting as capacitors and a conductor unlike the traditional plastic circuit boards. The inventors found the problem inherent to silicon for the first time. Nobody is aware of the problem yet.
An imaginary example would be helpful to clarify the electrical crosstalk between the transmitting portion and the receiving portion. The imaginary example would be assumed to have a receiving portion including a PD and an amplifier and a transmitting portion including an LD and an LD-driving IC. The imaginary module does not belong to prior art. No actual prior module contains the PD-AMP receiving part and the LD-IC transmitting part. The present invention has a two step advantage from the state of art. Then, it is not easy to understand the problems which confronted the inventors of the present invention. The imaginary example is a virtual LD/PD module intervening between the present invention and the nearest prior art. The virtual module has the following defects.
FIG. 3 shows an inherent problem of a silicon substrate. In the figure, a silicon substrate 10 has a fiber, an optoelectronic device (PD or LD) and an electrical device chip 27 behind the optoelectronic device. The electrical device 27 is either a driving IC for an LD module or an amplifier IC for a PD module. An enlarged section of the electrical device 27 is shown at a righthand margin of FIG. 3. An insulating film 29 and an electrode pad 28 are formed upon the silicon substrate 10. The electrical device 27 is fitted upon the electrode pad 28.
The silicon substrate 10 is an n-type or a p-type semiconductor which has low resistivity and allows electric current to flow well. The thin insulating film 29 prohibits DC electric current from flowing. However, high frequency AD current can jump over the insulating film 29 via an electrostatic capacitance. Optical communications employ high frequency (repetition rate) pulse signals. Such high frequency signals can pass through the thin insulating film 29. The high frequency transmitting signals travel in the silicon substrate, pass the thin insulating film by the electrostatic capacitance, reach the receiving part and induce noise (substrate-propagating crosstalk). This is the above-mentioned silicon inherent problem. The conductive silicon causes the problem. Print circuit boards made from the epoxy resin are entirely free from the substrate-conductive crosstalk owing to the high resistivity.
Since silicon is not an insulator but a semiconductor, the assembly of the capacitor having the SiO2 film and the conductive silicon substrate allows the noise current of the transmitting part to propagate to the receiving part. The driving IC yields strong pulse signals. The signals pass the insulating film and the silicon substrate and arrive at the amplifier, which causes the electric crosstalk. Conventionally, electrical crosstalk is believed to be induced by electromagnetic waves. The inventors of the present invention were aware of the current-induced crosstalk which is caused by the current flowing in the silicon substrate.
FIG. 4 shows a mode of crosstalk propagation in an LD/PD module due to electric current again. The crosstalk is a serious problem which has not been pointed out yet. A thin insulating film 11 coats a silicon substrate 10. A middle dotted line 26 is a boundary. The right side is a transmitting portion B. The transmitting portion B has metallized patterns 16 and 20 and a driving IC 24 on the metallized pattern 16. The insulating film 11 makes capacitors C1 and C2 between the silicon substrate 10 and the patterns 16 and 20. The left side of the boundary line 26 is a light receiving portion C. The receiving portion C has metallized patterns 17 and 21 and an amplifier 25 on the metallized pattern 17. The insulating film 11 makes capacitors C3 and C4 between the silicon substrate 10 and the patterns 17 and 21.
The silicon substrate has some conductivity which admits electric current. In the silicon substrate, effective resistors R1, R2 and R3 are formed among the capacitors C1, C2, C3 and C4. The transmitting portion B is joined to the receiving portion C by the capacitors and the effective resistors. The transmitting signals have high frequency (high repetition rates). The impedance of the capacitors 1/j xcfx89 C is very small. The strong signal of the driving IC propagates via the capacitors and the resistors to the amplifier 25. The leak of the driving IC signal to the amplifier induces electric crosstalk. Namely, in addition to electromagnetic waves, the silicon substrate itself carries noise from the driving IC 24 on the transmitting portion B to the amplifier 25 on the receiving portion C. The inventors of the present invention found the electric crosstalk conveyed by the silicon substrate in PLC modules for the first time. The prior art {circle around (2)} may be effective for alleviating the spatial crosstalk carried by electromagnetic waves. But {circle around (2)} is incompetent to reduce the electric crosstalk propagating in the silicon substrate.
In practice, the silicon substrates which are employed in the PLC LD/PD modules have innegligible conductivity which increases the cross talk by allowing the receiving portion to couple with the transmitting portion electrically. The inventors insist that attention should be paid to the current coupling between the transmitting and receiving portions in the PLC type modules. Recent miniaturization of modules narrows the distance of fibers in the double fiber transmitting/receiving modules which utilize two fibers for bidirectional communications. Narrower fiber distance enhances the electrical (current) crosstalk stronger.
In FIG. 4, the driving IC 24 is very close to the amplifier (AMP) IC 25. Miniaturization of modules shortens the fiber distance and the IC distance more. The faster the transmitting speed of signals is, the stronger electromagnetic waves the driving IC emits. The high signal speed reduces the capacitor impedance 1/j xcfx89 C. These conditions cooperate to enhance the crosstalk. The transmitting speed of the prevalent systems is now 156 Mbps. The transmitting speed will rise via 622 Mbps up to 1.25 Gbps or 2.5 Gbps in near future. The high speed transmission will induce serious current coupled electrical crosstalk.
The fiber distance is another significant factor raising the crosstalk between the PD part and the LD part. Current prevailing networks have a wide core-core distance between the transmitting (sending) fiber (S) and the receiving fiber (R) of 6.25 mm, which is 50 times as large as a fiber diameter (125 xcexcm). Some networks, however, try to shorten the fiber distance down to 4.5 mm (36 times of fiber diameter) or less than 1 mm. Extremely narrow distances of 0.75 mm (6 times of fiber diameter) or 0.25 mm (twice of fiber diameter) are also proposed as the fiber core-core distance. The shortening of the fiber distance forces optoelectronic devices to approach nearer and nearer, which raises the crosstalk.
As described hitherto, the crosstalk between the transmitting portion and the receiving portion includes optical crosstalk and electrical crosstalk. The electrical crosstalk seems to have been confused with electromagnetic crosstalk. However, the electrical crosstalk also includes current-induced crosstalk which is caused by the current flow in the silicon substrate and the induction current via the substantial capacitors built by thin insulating films on the silicon substrate. One purpose of the present invention is to provide a PLC type LD/PD module capable of curbing the current induced crosstalk. Another purpose of the present invention is to provide an inexpensive, small-sized LD/PD module by suppressing the crosstalk between the LD part and the PD part or among a plurality of PD parts.
An LD/PD module of the present invention includes a main silicon substrate with a cavity and a plateau side by side, an insulating auxiliary substrate embedded in the cavity of the main substrate, a transmitting part mounted upon the plateau of the main silicon substrate, a receiving part mounted on the auxiliary insulating substrate. Namely, the transmitting part is directly sustained by the silicon main substrate but the receiving part is indirectly sustained via the auxiliary insulation substrate (submount). The transmitting part includes at least an LD chip. The transmitting part optionally includes an LD driving IC. The receiving part includes at least a PD chip and optionally includes an amplifier for amplifying the photocurrent of the PD. The main silicon substrate has a light guiding medium coupled to the LD of the transmitting part and another light guiding medium coupled to the PD of the receiving part. The gist of the present invention is the asymmetry of mounting the transmitting part directly to the silicon substrate for heightening the coupling efficiency and mounting the receiving part on the insulating substrate for suppressing the current-induced crosstalk.
The planar lightguide circuit (PLC) type modules making use of single crystal silicon substrates have been contrived for eliminating the alignment process which consumes much time and labor. Silicon is a unique material accompanied by matured photolithography and highly precise finishing technology. The photolithography can make any fine, complex structures, for example, holes, hills, grooves or cavities on single crystal silicon substrates.
The silicon substrate allows us to mount an LD and a PD at the optimum spots by marking on the substrate and to fit fibers at exact position by inserting them into V-grooves. This is an advantage of the silicon single crystal substrate. The LD/fiber junction requires accuracy of an order of 1 xcexcm. Thus, the alignment of the LD/fiber should be assigned to the silicon substrate.
However, the receiving portion admits wider tolerance than the transmitting portion. The rigorous alignment is unnecessary to the PD/fiber junction. Thus, the present invention tries to mount the receiving part on the insulating submount.
Insulating substrates are inferior to silicon in positioning parts, since anisotropic etching is impossible. However, the PD/fiber joint requires not so rigorous alignment, because the PD has a wide sensitive region. The required tolerance for the receiving part is far wider than the tolerance for the transmitting part. Thus, this invention mounts the receiving part on the insulating substrate, relying upon the moderate tolerance of the PD/fiber junction.
Investigating the planar lightguide circuits, the Inventors of the present invention noticed that the rigorous tolerance for the LD/fiber junction should be intact but the tolerance of the PD/fiber joint can be alleviated since the PD has a wide sensitive region. Thus, the PD/fiber junction should not be necessarily mounted on the silicon single crystal substrate. The present invention has originated from the change of the common sense of the PD/fiber junction.
FIG. 8 shows the principle of cutting the current flow by the insulating substrate on which this invention is based. The right half is a transmitting portion which is directly mounted upon a silicon single crystal substrate 30. An LD driving IC 44 is coupled in AC mode via the capacitance of an insulating film 36 to the silicon substrate 30. The left half is a receiving part which is mounted upon an insulating auxiliary substrate 46.
The current flowing in the silicon substrate 30 is stopped by the insulating thick substrate 46. The LD current cannot arrive at the receiving part. The insulating submount 46 forbids a current-induced crosstalk.